Dr. Afsaneh Rabiei has invented an ultra-strong and lightweight
composite
metal foam. Credit: College of Engineering at North Carolina State
University

Doctoral student Lakshmi J. Vendra (left) and senior Judy Brown
(right)
help Dr. Rabiei with her research. Credit: College of Engineering at
North
Carolina State University

She's fluent in three languages. She's studied everywhere from
Tehran
to Tokyo to Cambridge, Mass. And she's invented a space-age material so
light and strong that it could revolutionize everything from vehicle
bumpers
to armor to biomedical devices.

The ultra-high-strength composite metal foam created by Afsaneh
Rabiei
is a highlight of a well-traveled career during which the researcher
has
tried to learn everything she can about advanced materials. The result:
a brand new material that can save energy and lives.

"Basically, it is a new material for all sorts of safety devices,"
said
Rabiei, associate professor of mechanical and aerospace engineering at
North Carolina State University.

Rabiei's invention isn't the first metal foam, but she says it's
the
strongest. The main weakness of existing metal foams is the varying
sizes
of their cells --- tiny pockets of space inside the material. Instead,
Rabiei used cells of standard sizes and combined them with a metallic
matrix
to support the cell walls.

A light, strong material

Rabiei has been working on high-performance materials for more
than
20 years. She studied materials science and engineering while getting
her
undergraduate degree at Sharif University of Technology in Tehran,
Iran,
in 1986. After spending some time in industry --- getting more
experience
in casting, welding and materials testing --- she returned to academia
and obtained her Ph.D. in 1997 from The University of Tokyo. Her work
on
the metal foams began when she was a post-doctoral researcher at
Harvard
University.

Along the way, she's become fluent in English, Japanese and
Persian
and written four books and dozens of other publications. The National
Science
Foundation awarded her a CAREER award in 2003, and her goal with the
funding
was to develop a light, strong material that could be used saving lives
and energy in the aerospace, medical, automotive and other industries.

After five years of work --- with a group of her students --- she
has
the results. Rough traffic accident calculations show that by inserting
two pieces of her composite metal foam behind the bumper of a car
traveling
28 mph, the impact would feel the same to passengers as impact
traveling
at only 5 mph.

Applications: airplanes, boats

The results are most striking when the material is tested in a
lab.
The test itself is exciting: a high-powered machine smashes a piece of
steel foam straight down into the base plate of the machine, and then
does
the same thing with a piece of bulk steel.

When she examines the base plates under both samples,
there&rsquo;s
a clear indentation left under the bulk steel sample, while the plate
under
the foam shows no indentation. The test shows how the foam absorbed the
energy and protected the plate, while the steel simply transferred it
to
the base plate with no protection.

And since the bulk steel is three times heavier than the steel
foam,
it's easy to see how the foam could attract car manufacturers looking
for
a bumper that will improve safety and gas mileage.

Rabiei sees plenty of uses for her invention, including in
airplanes,
boats, and structures that need impact protection with maintaining low
weight. It's this high strength-to-density ratio --- defining a
material
that's both strong and light --- that makes Rabiei's foam unique.

"This material showed a much higher strength-to-density ratio than
any
metal foam that has ever been reported," she said.

http://www.urweb.tv/UNC/metalfoam.html

High Performance Composite Metal Foam

by Terry Bray

Researchers at NCSU are joining forces with innovative companies
to
introduce new and enhanced products to the marketplace. Research and
the
commercialization of emerging technologies are key synergies of NCSU's
mission. In support of that mission, the Office of Technology Transfer
(OTT) promotes collaboration with industry partners to move university
discoveries from the laboratory to the marketplace. We are pleased to
promote
and foster beneficial relationships between academia and industry. NCSU
is currently seeking an industry partner to license a series of new
methods
for creating high-strength, ultra-light composite metallic foams that
show
5 to 6 times greater strength to density ratio and over 7 times higher
energy absorption than that of currently available metallic foams.

Researchers at NC State have developed, processed, and tested a
new
high-strength ultra-light material that combines the advantages of
metal
matrix composites with metallic foams. Dr. Afsaneh Rabiei has produced
a new generation of metal foams showing 5 to 6 times greater strength
to
density ratio and over 7 times higher energy absorption than that of
currently
available metallic foams. As a result, the energy absorption of these
materials
is estimated to be over 80 times greater than the bulk material from
which
the foam is made. Dr. Rabiei was interested in maintaining the
advantages
of metallic foams (excellent rigidity/ weight ratio, durability,
isotropic
absorption of energy at low and constant stress) while improving the
mechanical
properties under cyclic compression loading. The performance advantages
of this metal foam are based on improving foam cell structure and
reinforcing
the cells with a metallic matrix. The resulting novel, closed-cell,
metallic
foam composite is made from preform hollow metallic spheres and
exhibits
a strength of over 130 MPa in compression. The densification for the
new
foam occurs at strains of approximately 50-65%.

Dr. Afsaneh Rabiei serves as an Assistant Professor of Mechanical
and
Aerospace Engineering and as anAssociate Faculty Member of Biomedical
Engineering
at NC State since Aug. 2000. Dr. Afsaneh Rabiei received her Ph.D. in
advanced
materials at Research Center for Advanced Science and Technology, The
University
of Tokyo, Japan in 1997 within the area of mechanics and nondestructive
evaluation of metal matrix composites. Her prior working experience
includes
over 8 years of industrial experiences in materials science and
processing
including casting, welding and nondestructive testing. She received her
B.S. from the Department of Metallurgy and Material Science at Sharif
University
of Technology in Tehran, Iran in 1986. She worked at Harvard University
as a post doctoral researcher from 1997 until 2000.

Abstract --- The present invention is directed to
composite
metal foams comprising hollow metallic spheres and a solid metal
matrix.
The composite metal foams show high strength, particularly in
comparison
to previous metal foams, while maintaining a favorable strength to
density
ratio. The composite metal foams can be prepared by various techniques,
such as powder metallurgy and casting.

[0004] Metallic foams are a class of materials with very low
densities
and novel mechanical, thermal, electrical, and acoustic properties. In
comparison to conventional solids and polymer foams, metal foams are
light
weight, recyclable, and non-toxic. Particularly, metal foams offer high
specific stiffness, high strength, enhanced energy absorption, sound
and
vibration dampening, and tolerance to high temperatures. Furthermore,
by
altering the size and shape of the cells in metal foams, mechanical
properties
of the foam can be engineered to meet the demands of a wide range of
applications.

[0005] Various methods are presently known in the art for
preparing
metallic foams. According to one method, metal powders are compacted
together
with suitable blowing agents, and the compressed bodies are heated
above
the solidus temperature of the metal matrix and the decomposition
temperature
of the blowing agent to generate gas evolution within the metal. Such
"self-expanding
foams" can also be prepared by stirring the blowing agents directly
into
metal melts. Metallic foams can also be prepared as "blown foams" by
dissolving
or injecting blowing gases into metal melts. Metallic foams can also be
prepared by methods wherein gasses or gas-forming chemicals are not
used.
For example, metal melts can be caused to infiltrate porous bodies
which
are later removed after solidification of the melt, leaving pores
within
the solidified material.

[0006] Metallic foams have been shown to experience fatigue
degradation
in response to both tension and compression. Plastic deformation under
cyclic loading occurs preferentially within deformation bands, until
the
densification strain has been reached. The bands generally form at
large
cells in the ensemble, mainly because known processes for producing
these
materials do not facilitate formation in a uniform manner. Such large
cells
develop plastically buckled membranes that experience large strains
upon
further cycling and will lead to cracking and rapid cyclic straining.
As
a result, the performance of existing foams has not been promising due
to strong variations in their cell structure (see Y. Sugimura, J.
Meyer,
M. Y. He, H. Bart-Smith, J. Grenstedt, & A. G. Evans, "On the
Mechanical
Performance of Closed Cell Al Alloy Foams", Acta Materialia, 45(12),
pp.
5245-5259).

[0007] In the production of closed cell metallic foams, one
obstacle
is the inability to finely control cell size, shape, and distribution.
This makes it difficult to create a consistently reproducible material
where the properties are known with predictable failure. One method for
creating a uniform closed cell metallic foam is to use prefabricated
spheres
of a known size distribution and join them together into a solid form,
such as through sintering of the spheres, thereby forming a closed cell
hollow sphere foam (HSF).

[0008] Hollow metal spheres, such as those available from
Fraunhofer
Institute for Manufacturing and Advanced Materials (Dresden, Germany),
can be prepared by coating expanded plastic spheres (e.g., polystyrene)
with a powdered metal suspension. The spheres are then placed into a
mold
and are heated to pyrolize the polystyrene and powder binder, and to
sinter
the metal powder into a dense, solid shell. Metal foams previously
prepared
through sintering of such hollow metal spheres are plagued by low
strength.
Foams prepared by sintering metal spheres made of stainless steel, when
under compression, have been shown to undergo densification at a stress
of approximately 2 to 7 MPa, reaching a strain of over 60%.

[0009] Accordingly, it is desirable to have metallic foams wherein
cell
size, shape, and distribution are controllable, and wherein high
strength
is exhibited. Such goals are achieved by the composite metal foams of
the
present invention and the methods of preparation thereof.

SUMMARY OF THE INVENTION

[0010] The present invention is a composite metallic foam
comprising
hollow metal spheres and a solid metal matrix. The foam exhibits low
density
and high strength. Generally, the composite metallic foam is prepared
by
filling in the spaces around the hollow metallic spheres, thus creating
a solid matrix. Such preparation can be by various methods, including
powder
metallurgy techniques and casting techniques. The composite metallic
foams
of the invention have unique properties that provide use in multiple
applications,
such as marine structures, space vehicles, automobiles, and buildings.
The foams are particularly useful in applications where weight is
critical
and vibration damping, as well as energy absorption, are useful, such
as
blast panels for military applications and crumple zones for automotive
crash protection. The application of the foams can also be extended
into
biomedical engineering as medical implants and even to civil
engineering
for earthquake protection in heavy structures.

[0011] The composite metal foams of the invention, partly due to
their
controlled porosity (through use of preformed hollow metallic pieces)
and
foam cell wall support (through addition of a metal matrix surrounding
the hollow metallic pieces), exhibit highly improved mechanical
properties,
particularly under compression loading. Accordingly, the strength of
the
inventive composite metal foams is many times higher than other
metallic
hollow sphere foams. Furthermore, the energy absorption of the
inventive
foams is much greater than the bulk material used in the foams (on the
order of 30 times to 70 times greater), while the inventive foams also
maintain a density well below that of the bulk materials.

[0012] In one aspect of the invention, there is provided a
composite
metal foam comprising a plurality of hollow pieces (preferably hollow
metallic
pieces) and a metal matrix generally surrounding the hollow pieces. The
hollow pieces and the matrix can be comprised of the same or different
materials. In one embodiment, the hollow pieces are metallic spheres
comprising
steel, and the metal matrix comprises steel. In another embodiment, the
metal matrix comprises aluminum, while the hollow spheres comprise
steel.

[0013] According to another aspect of the invention, there is
provided
a method of preparing a composite metallic foam comprising placing a
plurality
of hollow metallic pieces in a mold and filling the spaces around the
hollow
metallic pieces with a metal matrix-forming material. The method can be
carried out through the use of various techniques, such as powder
metallurgy
or metal casting.

[0014] In one particular embodiment according to this aspect of
the
invention, the method comprises the following steps: arranging a
plurality
of hollow metallic pieces in a mold; filling the spaces around the
hollow
metallic pieces with a matrix-forming metal powder; and heating the
mold
to a sintering temperature, thereby forming a solid metal matrix around
the hollow metallic pieces. Various packing techniques, such as
vibrating
the mold according to a specific frequency, or varying frequencies, can
be used for maximizing packing density of the metallic pieces within
the
mold. Further, such techniques can also be used during the step of
filling
the spaces around the hollow metallic pieces to facilitate movement of
the metal powder through the mold and around the hollow metallic
pieces.

[0015] The method can further comprise applying pressure to the
hollow
metallic pieces and the matrix-forming metal powder within the mold, as
would commonly be done in powder metallurgy techniques. Such
compression
within the mold can be carried out for the duration of the sintering
step
of the method.

[0016] According to another embodiment of the invention, the
method
comprises the following steps: arranging a plurality of hollow metallic
pieces in a mold; casting a matrix-forming molten metal into the mold,
thereby filling the spaces around the hollow metallic pieces; and
solidifying
the liquid metal, thereby forming a metal matrix around the hollow
metallic
pieces. As noted above, various packing techniques, such as vibrating
the
mold, can be used for maximizing packing density of the metallic pieces
within the mold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical image providing a cross-sectional view
of
a 3.7 mm hollow metallic sphere useful according to the present
invention;

FIG. 2 is a cross-sectional view of a composite metal foam
of
the invention comprising hollow steel spheres surrounded by a metal
matrix
formed by powder metallurgy using steel powder;

FIG. 3 is a detailed cross-sectional view of a composite
metal
foam formed by powder metallurgy, according to one embodiment of the
invention,
comprising hollow steel spheres surrounded by a steel matrix;

FIG. 4 is a cross-sectional view of the composite metal
foam
shown in FIG. 3 providing an even greater detailed view of the metal
matrix;

FIG. 5 is a SEM image of the composite metal foam of FIG. 2
showing
a cross-section of two steel spheres in contact with each other and the
steel matrix filling the spaces around the spheres;

FIG. 6 is another SEM image of the composite metal foam of
FIG.
2 showing a cross-section of two spheres not in contact and the steel
matrix
filling the spaces between and around the spheres;

FIG. 7 is a three-dimensional drawing of a permanent
casting
mold useful in one embodiment of the invention;

FIG. 8 is a cross-sectional view of a permanent casting
mold
useful in one embodiment of the invention;

FIG. 9 is a cross-sectional view of a composite metal foam
of
the invention formed by casting molten aluminum around hollow steel
spheres;

FIG. 10 is a SEM image of a cross-section of a composite
metal
foam of the invention showing an aluminum matrix between two hollow
steel
spheres;

FIG. 11 is a detailed view of the SEM image from FIG. 10
showing
the interface between the aluminum matrix and the steel wall of the
hollow
sphere;

FIG. 12 is a SEM image of a cross-section of a composite
metal
foam of the invention formed by casting an aluminum matrix around
hollow
steel spheres and shows (a) four spheres embedded in the matrix with a
visible void at the interface of two spheres, and (b) a detail view of
the aluminum matrix showing the different phases present;

FIG. 13 is a cross-sectional optical image of three
composite
metal foams prepared according to various embodiments of the invention;

FIG. 14 is a chart of the stress-strain curves of composite
metal
foams according to various embodiments of the invention under monotonic
compression;

FIG. 15 shows a stainless steel composite metal foam according
to
one embodiment of the invention both before and after compression
testing
with 60% strain;

FIG. 16 is a chart showing a curve of strain versus number
of
cycles during a compression fatigue test of a cast composite metal foam
according to one embodiment of the invention; and

FIG. 17 shows optical images of a cast composite metal,
according
to one embodiment of the invention, before and during a compression
fatigue
test.

DETAILED DESCRIPTION OF THE INVENTION

[0034] The invention now will be described more fully hereinafter
with
reference to the accompanying drawings, in which preferred embodiments
of the invention are shown. These embodiments are provided so that this
disclosure will be thorough and complete, and will fully convey the
scope
of the invention to those skilled in the art. Indeed, the invention may
be embodied in many different forms and should not be construed as
limited
to the embodiments set forth herein; rather, these embodiments are
provided
so that this disclosure will satisfy applicable legal requirements. As
used in the specification, and in the appended claims, the singular
forms
"a", "an", "the", include plural referents unless the context clearly
dictates
otherwise.

[0035] The composite metallic foam of the present invention
combines
the advantages of metal matrix composites with the advantages of
metallic
foams to provide higher strength metallic foams of controlled porosity.
The inventive metal foams generally comprise a plurality of hollow
metallic
pieces and a metal matrix filling the spaces around the metallic
pieces.

[0036] Metal matrix composites are generally understood to be
metals
that are reinforced with various materials. Such materials can include
natural or synthetic fibers or particulate matter. Materials
particularly
useful include boron, silicon carbide, graphite, alumina tungsten,
beryllium,
titanium, and molybdenum. Fibers may be continuous filaments or
discontinuous
fibers. Examples of natural discontinuous fibers include hair or
whiskers.
The reinforcements, of which the above are only non-limiting examples
thereof,
can be chosen for specific purposes, such as increasing stiffness,
strength,
heat resistance, and wear resistance. Combining the advantages of metal
matrix composites with the advantages of metal foams results in the
composite
metal foams of the invention, which exhibit increased strength, as well
as additional beneficial properties as discussed herein.

[0037] The composite metal foams of the invention comprise hollow
pieces.
In a particular embodiment of the invention, the hollow pieces are
spherical
in shape (i.e., "hollow spheres"). While such a shape is particularly
useful,
the hollow pieces comprising the metal foam can also take on other
geometric
shapes as could be beneficial for imparting improved properties to the
finished composite metal foam. For simplicity, the hollow pieces used
in
the invention are described herein by the particular spherical
embodiment.
However, description of the hollow pieces as spheres is not intended to
limit the scope of the hollow pieces, which can take on other shapes.

[0038] The hollow spheres used in the composite metal foams of the
invention
can comprise any material that would be useful for providing strength
in
an overall composite foam of the invention and can withstand the
preparation
process, such as powder metallurgy or casting, as described herein. In
one preferred embodiment, the hollow spheres are metallic.

[0039] Hollow metallic spheres, according to the invention, can
comprise
any metal generally recognized as being useful for preparing metal
foams,
metal matrix composites, or other metal components useful in various
industries,
such as automotive, aerospace, construction, safety materials, and the
like. Particularly useful are metals commonly used in applications
where
lightweight materials, or materials exhibiting relatively low density,
are desirable. For example, the hollow spheres can comprise iron (and
alloys
thereof), aluminum, titanium, nickel, ceramics, such as alumina and
silica
carbide, and the like. The metals comprising the hollow spheres can be
a single, essentially pure metal; however, the term metal, as used
herein
to describe the components of the composite metal foams of the
invention,
can also refer to metal mixtures, including alloys, intermetallic
compounds,
such as titanium aluminide, and the like. Further, the metals can
include
trace components as would be recognizable as being beneficial, as well
as non-detrimental trace impurities. In one particularly preferred
embodiment,
the hollow metallic spheres are comprised of steel, such as stainless
steel
or low carbon steel. The composition of one type of low carbon steel
and
one type of stainless steel (316L stainless steel) useful in particular
embodiments of the invention are provided in Table 1. TABLE-US-00001
TABLE
1 Exemplary Metal Compositions for Hollow Metallic Spheres Type
Composition
Low Carbon Steel: <0.007% carbon and 0.002% oxygen; remaining
balance
iron 316L Stainless Steel: 0.03% carbon, 0.3% oxygen, 17% chromium, 13%
nickel, 0.9% silicon, 0.2% manganese, 2.2% molybdenum, and remaining
balance
iron

[0040] The average size of the hollow metallic spheres can vary
depending
upon the desired physical properties of the finished composite metal
foam.
Average size of the spheres is generally evaluated in terms of sphere
diameter.
When considering the physical properties of the finished composite
metal
foam, though, sphere wall thickness must also be considered.
Accordingly,
assuming sphere wall thickness remains unchanged, the use of spheres
having
a greater average diameter would be expected to result in a finished
composite
metal foam of lower density than if spheres of smaller average diameter
are used. The average diameter is also limited by the size and
dimensions
of the finished composite metal foam. For example, if the desired
finished
composite metal foam is a metal sheet having a 25 mm thickness, the
hollow
metallic spheres would necessarily need to have an average diameter of
less than 25 mm.

[0041] The hollow metallic spheres used in the invention generally
have
an average diameter of about 0.5 mm to about 20 mm. Preferably, the
spheres
have an average diameter of about 1 mm to about 15 mm, more preferably
about 1.5 mm to about 10 mm, still more preferably about 2 mm to about
8 mm, most preferably about 2.5 mm to about 6 mm. In one particular
embodiment,
hollow metallic spheres having an average diameter of about 3 mm to
about
4 mm (nominally about 3.7 mm) have been used to prepare the composite
metal
foam of the invention. Depending upon the desired properties of the
composite
metal foam, other sphere sizes can also be used.

[0042] As noted above, sphere size is also described by the sphere
wall
thickness, which similarly affects the physical properties of the
finished
composite metal foams. For example, assuming sphere average diameter is
unchanged, the use of spheres having a lesser average wall thickness
would
be expected to result in a finished composite metal foam of lower
density
than if spheres of greater average wall thickness are used.
Accordingly,
in one embodiment of the invention, it is desirable to minimize wall
thickness.
If wall thickness is too minimal, though, strength of the finished
composite
metal foams can be compromised. It is therefore beneficial to use
spheres
wherein the ratio of wall thickness to average sphere diameter is in a
range where density of the finished composite metal foam is minimized
but
overall strength of the composite metal foam is not appreciably
sacrificed.

[0043] The hollow metallic spheres of the invention generally have
an
average wall thickness that is about 1% to about 30% of the average
diameter
of the spheres. Preferably, the average sphere wall thickness is about
1% to about 15% of the average sphere diameter, more preferably about
1.5%
to about 10%, still more preferably about 2% to about 8%, and most
preferably
about 2.5% to about 7% of the average sphere diameter. In one
particular
embodiment, the average sphere wall thickness is about 5% of the
average
sphere diameter. A cross-section of a hollow metallic sphere, such as
useful
according to the invention is shown in FIG. 1 (note that the sphere in
the figure has not been cut through the diameter of the sphere). As
seen
in the Figure, the sphere walls have a generally uniform thickness.
This
is particularly advantageous in that composite metal foams, according
to
the invention, can be prepared to uniform porosity, said porosity being
easily adjustable by use of hollow metallic spheres of a desired
average
diameter and average wall thickness.

[0044] Preferentially, the percentage and size of porosities in
the
sphere walls are minimized to increase stability of the spheres during
processing of the foams. For example, when casting techniques are used
in preparing the composite metal foams, minimizing sphere wall porosity
decreases the possibility of the matrix-forming molten metal
penetrating
the cavities of the spheres. Such penetration should be avoided as
filling
of the cavities could reduce the overall pore volume of the composite
metal
foam, unnecessarily increasing the overall density of the foam. In one
embodiment, sphere wall porosity is less than about 12%. Preferably,
sphere
wall porosity is less than about 10%, more preferably less than about
8%,
most preferably about 5% or less.

[0045] In addition to the hollow metallic pieces, the composite
metal
foam of the invention also comprises a matrix surrounding the hollow
metallic
pieces. The matrix generally comprises a metal, and the type of metal
comprising
the matrix can be varied depending upon the technique used in preparing
the composite metal foam of the invention.

[0046] According to one embodiment, the metal comprising the
matrix
can be the same metal type comprising the hollow metallic pieces.
According
to another embodiment, the metal comprising the matrix is a different
metal
type than that comprising the hollow metallic pieces. Preferably, the
metal
matrix includes a metal that is generally lightweight but still
exhibits
good strength attributes. The use of such metals is beneficial for
maintaining
a high strength to density ratio in the finished composite metal foam
of
the invention. As before, the metal comprising the metal matrix can be
an essentially pure single metal or can be a mixture of metals. In one
particular embodiment, the metal matrix comprises steel. In another
embodiment,
the metal matrix comprises aluminum.

[0047] Matrix composition may at least partially be dependant upon
the
method of preparation of the composite metal foam. The composite metal
foams of the invention can be prepared through various techniques known
in the art. While the use of such techniques would not be readily
apparent
for preparing metal foams, one of skill in the art, with the benefit of
the present disclosure, could envision similar techniques which could
be
used in preparing the composite metal foams of the invention. Such
further
techniques are also encompassed by the present invention.

[0048] According to one embodiment of the invention, there is
provided
a method for preparing a composite metal foam by powder metallurgy.
According
to this method, the hollow metallic spheres are first placed inside a
mold.
At this point, it should be noted that the composite metal foam can be
prepared directly in the final desired shape through use of a mold
designed
to provide the desired shape. Alternately, the composite metal foam may
be prepared as a "stock" piece (e.g., a large rectangle) and then be
cut
to the desired final shape. The size of the composite metal foam
prepared
according to this embodiment of the invention is generally limited by
the
size of the mold.

[0049] The metallic spheres are preferentially arranged in the
mold
to be in a specific packing arrangement. Desirably, the packing
arrangement
is such that the metallic spheres are in their most efficient packing
density
(i.e., most closely packed). As such, the open space between the
spheres
is minimized, and the number of spheres arranged in the mold is
maximized.
In this packing arrangement, the porosity of the finished composite
metal
foam is maximized, which correlates into a minimized density.

[0050] The arrangement of the metallic pieces in the mold can be
facilitated
through mechanical means, such as vibrating the mold. In embodiments
where
metallic spheres are used, vibration is particularly useful as the
spheres
tend to "settle" into a most preferred packing density. For example,
such
vibration can be performed using an APS Dynamics model 113 shaker and
an
APS model 114 amplifier with a General Radio 1310-B frequency
generator.
Vibrating at specific frequencies may be beneficial for facilitating a
closest packing density or for minimizing the time necessary to obtain
such a packing density. Vibrating time may vary depending upon the size
of the mold, the average size of the hollow metallic pieces, the
average
size of the metal powders, and the frequency of the vibration.
Generally,
vibrating can be performed for a period of time up to about 12 hours,
although
longer or shorter time periods may be necessary or sufficient. In one
particular
embodiment, vibrating is performed for a period of time ranging between
about 30 seconds and about 4 hours, preferably about 1 minute to about
3 hours, more preferably about 5 minutes to about 2 hours.

[0051] Similarly, other mechanical means can be used for
facilitating
packing density. Computer modeling could also be used to determine
optimum
packing techniques, including establishing sphere sizes most useful for
optimum packing densities in light of mold size and shape. Where
computer
modeling is used, automated packing of the spheres could be beneficial
for arranging the spheres in a maximized packing density. Further, in
embodiments
where hollow metallic pieces having a non-spherical geometry are used,
different mechanical means could be used for establishing the most
efficient
packing density of the metallic pieces given their geometries.

[0052] Once the hollow metallic spheres have been arranged in the
mold,
a matrix-forming metal powder is introduced into the mold, filling the
spaces around the hollow metallic spheres. Again, mechanical means,
such
as vibrating, can be used to facilitate movement of the metal powder
around
the hollow metallic spheres, preferentially completely filling any
voids
within the mold. Multiple rotations of adding powder and applying
mechanical
means to move the powder into the voids between spheres within the mold
could be used to ensure complete filling of the mold. Further, it may
be
beneficial, particularly when filling large molds, or molds of complex
shape, to alternate introduction of the spheres and the matrix forming
powder into the mold to ensure complete filling of the spaces between
the
hollow metallic pieces.

[0054] Choice of metal powder can depend upon the desired physical
properties
of the composite metal foam. Further, choice of metal powder can be
limited
by such characteristics as particle size and flow characteristics. For
example, electrostatic interactions can limit the flow of some powder
types
leading to agglomeration and incomplete filling of the voids between
the
hollow metallic spheres.

[0055] Choice of metal powder can also be limited by chemical and
physical
changes in the matrix material brought about by sintering. For example,
it is known that the strength of sintered steel increases with
increasing
carbon content, up to a bout 0.85% carbon (see ASM Metals Handbook,
9.sup.th
Edition, Vol. 7, "Powder Metallurgy", American Society for Metals,
1984,
which is incorporated herein by reference). Beyond this, a network of
free
cementite begins to form at the gain boundaries. Additionally, it has
been
shown that for similar sintering conditions, shrinkage decreases with
increasing
carbon content up to 8%, at which no shrinkage was noted (see, N.
Dautzenberg,
Powder Metallurgy International, vol. 12, 1971 and Dautzenberg and
Hewing,
Powder Metallurgy International, vol. 9, 1977, both of which are
incorporated
herein by reference).

[0056] Further considerations in choosing the metal matrix-forming
powder
arise from the possible formation of unsuitable intermetallic compounds
during sintering. Such formation can be prevented, to some extent, by
controlling
sintering conditions. For example, when using an aluminum powder
matrix-forming
material with hollow steel spheres, diffuision of matrix material into
the spheres and the formation of a brittle intermetallic phase may
occur,
particularly with slow process and prolonged exposure of the
combination
of iron and aluminum at higher temperatures.

[0057] The metal powder is preferentially of a particle size
capable
of achieving a favorable packing system for maximizing matrix density.
For example, in one embodiment, aluminum powder is used, the powder
being
a 98% pure mixture of the following components: 75% H-95 Al powder
(about
100 micron particle size); 14% H-15 Al powder (about 15 micron particle
size); and 11% H-2 Al powder (about 2 micron particle size). Such
powders
are available commercially from vendors, such as Valimet, Inc.
(Stockton,
Calif.). A powder composition, such as described above, is close to the
ideal 49:7:1 ratio to achieve an optimum trimodal packing system for
greater
matrix density. In another embodiment, Ancorsteel-1000 C iron powder is
used. The powder is sieved to 81.3% -325 mesh (44 micron) powder and
18.7%
-400 mesh (37 micron) powder. Ancorsteel-1000 C powder is commercially
available from ARC Metals (Ridgway, Pa.). In further embodiments,
powders
of an essentially uniform particle size, or of various particles sizes,
can be used for maximizing matrix density. For example, powders having
particle sizes most favorable for achieving an optimum bimodal packing
system could also be used.

[0058] In one embodiment, the metal powders used as a
matrix-forming
metal powder in the invention have an average particle size of about 1
.mu.m to about 200 .mu.m. Preferably, the metal powder has an average
particle
size of about 10 .mu.m to about 100 .mu.m, more preferably about 15
.mu.m
to about 75 .mu.m, most preferably about 20 .mu.m to about 50 .mu.m.

[0059] Metal powders, such as those described above, can be used
alone
as the matrix forming metal powder. Alternately, further additional
components
can be combined with the metal powder. For example, in one embodiment
of
the invention, the metal powder further includes carbon in the form of
-300 mesh crystalline graphite to further increase the strength of the
low carbon steel matrix, as described above.

[0060] Further reinforcement agents can also be added to the metal
powder
prior to introduction of the powder into the mold. For example, natural
or synthetic fibers or particulate matter could be mixed with the metal
powder, or added into the mold, to provide additional benefits, such as
increased strength or heat resistance.

[0061] Once the spaces around the hollow metallic spheres have
been
filled with the matrix-forming metal powder, the mold is heated to a
sintering
temperature, the appropriate temperature being dependant upon the
composition
of the powder and the composition of the metallic spheres. Where the
metal
powder has a sintering temperature well below the sintering temperature
of the metallic spheres, the lower temperature may be used, thereby
sintering
the metal powder and forming a solid metal matrix around the hollow
metallic
spheres. When the metallic spheres and the metal powder comprise the
same
metal, or different metals having similar sintering temperatures, the
metal
powder and, to some extent, the metallic spheres are sintered, thereby
forming a solid metal matrix around the hollow metallic spheres.

[0062] In one embodiment, the metal powder is sintered with the
contents
of the mold under pressure, such as in a hot press. Pressure values can
vary depending upon the mechanical and physical properties of the
spheres.
Further, sphere size can also affect the applied pressure range.
Acceptable
pressure ranges can be calculated based upon the yield strength of the
hollow sphere and the permissible load that can be applied to the
spheres
without any permanent deformation of the spheres.

[0063] According to one embodiment, sintering is conducted without
application
of external pressure. In this embodiment, thermal expansion of the
spheres
during sintering and the resulting localized pressure around the
spheres
were used to facilitate pressing of the powder into the spaces between
the spheres. The results show minimal porosity in the matrix of the
composite
metal foam after sintering.

[0064] Sintering temperature can vary depending upon materials
used
in the spheres and, particularly, in the matrix-forming metal powder.
In
one embodiment, where hollow steel spheres and aluminum powder are
used,
the sintering is performed at a temperature of about 630.degree. C. In
another embodiment, where hollow steel spheres and steel powder are
used,
the sintering is at a temperature of about 1200.degree. C. Preferably,
sintering is performed at a temperature sufficient to exceed the
solidus
temperature of the metal matrix-forming powder but remain below the
liquidus
temperature of the powder. Further, preferably, the sintering
temperature
does not exceed the solidus temperature of the hollow metallic spheres.
In one particular embodiment, sintering is performed at a temperature
of
between about 500.degree. C. and about 1500.degree. C., preferably
between
about 550.degree. C. and about 1400.degree. C., more preferably between
about 600.degree. C. and about 1300.degree. C.

[0065] Sintering time can also vary depending upon the materials
used
in the hollow metallic spheres and the metal matrix-forming powder.
Sintering
time also varies, however, based upon the relative size of the mold
(and
therefore the size of the sample being prepared). Larger molds
obviously
require a longer sintering time to ensure sintering completely through
the thickness of the sample. Likewise, smaller molds requires a lesser
sintering time. Size considerations in relation to sintering time
generally
follow guidelines similar to those previously provided in relation to
powder
metallurgy processes.

[0066] Sintering conditions are preferably optimized to achieve
improved
mechanical properties. In one preferred embodiment, a duplex cycle is
used
to provide improved mechanical properties due to different sintering
mechanisms
taking place at each temperature. Such a method generally comprises
cycling
temperature increase phases with temperature hold phases. In one
particular
embodiment, where a composite metal foam is prepared using steel
spheres
and steel powder, the sample is heated at 10 .degree. C./minute, held
for
30 minutes at 850.degree. C., heated at 5.degree. C./minute, held for
45
minutes at 1200.degree. C., and cooled to room temperature at
20.degree.
C./minute. In such cycles, the lower temperature portion assists in the
removal of oxides and impurities and helps bring the mold to thermal
equilibrium
to avoid gradient properties. Surface transport effects are most
prevalent
at lower temperature, so the bonds between particles are strengthened
without
densification of the matrix. At higher temperatures, strength is
increased
greatly as a result of the higher sintering rate due to greater atomic
motion. For both temperatures, rapid increases in strength are noted
for
times up to 30 minutes, where the rate begins to decrease.

[0067] FIG. 2 provides an optical, cross-sectional image of a
hollow
metallic foam according to the invention prepared by powder metallurgy
using hollow steel spheres of 3.7 mm average diameter and a sintered
steel
powder matrix. FIGS. 3 and 4 provide scanning electron microscopy (SEM)
images of a composite stainless steel foam prepared using a powder
metallurgy
technique, as described above. FIG. 3 shows the cross-section of intact
spheres, and FIG. 4 shows the sintered powder matrix completely filling
the space between the spheres. The bonding between the spheres and the
matrix is seen to be strong with no voids at the interface.

[0068] Returning to FIG. 2, the hollow metallic spheres show some
signs
of
uniform packing; however, it is desirable to further increase the
uniformity
and density of the packing of the spheres to create metal foams
exhibiting
more uniform properties and even lower densities. The benefits of
improving
uniformity and density of packing are further illustrated in FIGS. 5
and
6.

[0069] FIG. 5 provides a SEM image of a cross-section of two
spheres
in contact with one another. FIG. 6 provides a SEM image of a
cross-section
of two spheres not in contact, but with the metal matrix filling the
space
between the spheres. Increasing packing density of the spheres
increases
the contact between the spheres reducing the amount of free space
between
the spheres. Consequently, increased packing density reduces the amount
of metal matrix present in the foam, which generally leads to lower
densities,
without sacrificing strength. FIGS. 5 and 6 further illustrate the
ability
to reduce the density of the composite metal foam by using hollow
metallic
spheres having lesser wall thicknesses. This is particularly
illustrated
in FIG. 5, wherein the sphere in the lower portion has a noticeably
thinner
wall than the sphere in the upper portion of the figure. The presence
of
the metal matrix surrounding the hollow metallic spheres allow for
reducing
the wall thickness to lower density of the composite metal foam without
sacrificing strength.

[0070] According to another embodiment of the invention, there is
provided
a method for preparing a composite metal foam by casting. In one
embodiment,
which is described below, the composite metal foam is prepared by
permanent
mold gravity casting; however, various other casting methods, as would
be recognizable by one of skill in the art, could be used. Accordingly,
the present invention is not limited by the permanent mold casting
method
described herein but rather encompasses all casting methods that could
be recognizable as useful.

[0071] In one embodiment of a casting method according to the
invention,
the hollow metallic spheres are first placed inside the mold. The
hollow
spheres are preferably arranged inside the mold, such as through
vibrating,
to pack the spheres into a best attainable close packed density.
Vibration
methods and apparatus, as described above in relation to powder
metallurgy
methods, would also be useful according to this aspect of the
invention.
Once the spheres are packed in the mold, a matrix-forming liquid metal
is cast into the mold, filling the spaces around the hollow metallic
spheres.
The liquid metal is then solidified to form a solid metal matrix around
the hollow metallic spheres.

[0072] One embodiment of a mold useful in the casting method of
the
invention is illustrated in FIGS. 7 and 8, which show a
three-dimensional
view and a cross-sectional view, respectively, of an open atmosphere
gravity
feed permanent mold. The mold incorporates a sprue, runner, melt
filter,
and overflow riser. Carbon steel is a particularly preferred material
for
the mold allowing for repeated exposure to molten metal and high
pre-heating
temperatures.

[0073] In a mold, such as shown in FIGS. 7 and 8, liquid metal is
poured
into the sprue. The liquid metal then travels through the runner, rises
up through a slide gate and melt filter, fills the spaces between the
hollow
metal spheres, and flows up into the over-flow riser. Such a
"bottom-up"
filling approach allows the liquid matrix-forming metal to push out the
air as the metal fills the interstitial space between the hollow
metallic
spheres. The slide gate allows for easy de-molding after
solidification,
and the melt filter serves to remove any solid impurities or oxides in
the melt. The overflow riser feeds any shrinkage during aluminum
solidification.

[0074] In one particular embodiment, prior to introduction of the
matrix-forming
liquid metal, the mold and hollow spheres are pre-heated. Preferably,
the
pre-heat temperature is at least about equal to the casting temperature
of the matrix-forming liquid metal. For convenience, the matrix-forming
metal can be liquefied in the same furnace used for pre-heating the
mold
and spheres. The temperature of the mold and spheres should be at least
about equal to the casting temperature of the matrix-forming liquid
metal
in order to prevent premature solidification of the matrix before
complete
filling of the mold, including the spaces between and around the
spheres.
The pre-heat temperature can be greater than the casting temperature of
the matrix-forming liquid metal so long as the temperature does not
approach
the solidus temperature of the spheres.

[0075] In this method, the hollow metallic spheres and the
matrix-forming
metal comprise different metal compositions, the compositions being
distinguished
by a difference in their melting temperatures. Since the matrix-forming
metal is introduced to the mold in a molten state, it is necessary that
the hollow metallic spheres comprise a metal composition having a
melting
temperature greater than the melting temperature of the matrix-forming
metal composition. This avoids the possibility of melting of the hollow
metallic spheres during pre-heating or during introduction of the
matrix-forming
liquid metal melting into the mold.

[0076] Where the metal compositions comprise essentially pure
single
metals, the transition from solid to liquid generally can be described
as a single melting temperature. Where metal mixtures are used,
however,
the state transition becomes more complex and can be described with
reference
to the solidus temperature and the liquidus temperature. When an alloy
is heated, the temperature at which the alloy begins to melt is
referred
to as the solidus temperature. Between the solidus and liquidus
temperatures,
the alloy exists as a mixture of solid and liquid phases. Just above
the
solidus temperature, the mixture will be mostly solid with some liquid
phases therein, and just below the liquidus temperature, the mixture
will
be mostly liquid with some solid phases therein. Above the liquidus
temperature,
the alloy is completely molten.

[0077] The metal composition used as the matrix-forming liquid
metal
of the invention should have a melting point (or a liquidus
temperature)
that is below the melting point (or solidus temperature) of the metal
composition
comprising the hollow metallic spheres. Preferably, the melting
temperature
of the matrix-forming liquid metal is at least about 25.degree. C. less
than the solidus temperature of the metal composition comprising the
hollow
metallic spheres, more preferably at least about 40.degree. C. less,
most
preferably at least about 50.degree. C. less than the solidus
temperature
of the metal composition comprising the hollow metallic spheres.

[0078] In one embodiment, the hollow metallic spheres are
comprised
predominately of steel and the matrix-forming liquid metal is aluminum
or an aluminum alloy. For example, the hollow metallic sphere could
comprise
low carbon steel or 316L stainless steel, such as according to the
compositions
exemplified in Table 1. Likewise, the matrix-forming liquid metal could
comprise aluminum 356 alloy, such as according to the composition
exemplified
in Table 2. Aluminum 356 alloy is particularly useful due to its low
density,
high strength and stiffness, and ease of casting of the material.

[0079] Further reinforcement agents can also be added to the
matrix-forming
liquid metal prior to casting. For example, natural or synthetic fibers
or particulate matter could be mixed with the liquid metal, or added
into
the mold, to provide additional benefits, such as increased strength or
heat resistance.

[0080] Preferentially, the matrix-forming liquid metal is cast
into
the mold in such a manner as to facilitate complete filling of the
voids
around the hollow metallic spheres while avoiding disturbance of the
hollow
metallic spheres arranged within the mold. In some embodiments, it may
be useful to use screens, or other similar means, for maintaining the
arrangement
of the spheres within the mold. In addition to gravity casting, the
mold
may be subject to pressure differentials during the cast process. For
example,
in one embodiment, the mold may be pressurized. In another embodiment,
the mold may be under a vacuum.

[0081] Once the matrix-forming liquid metal has been cast into the
mold,
the liquid metal is solidified to form a solid metal matrix around the
hollow metallic spheres. Such solidification is generally through
cooling
of the mold, which can be through a.mu.mospheric cooling or through
more
controlled cooling methods.

[0082] A composite metal foam, according to one embodiment of the
invention,
prepared by casting an aluminum metal matrix around hollow low carbon
steel
spheres, is shown in FIG. 9. As can be seen in the figure, the closest
packing arrangement of the hollow spheres is somewhat disturbed by the
inflow of the liquid metal matrix. Nevertheless, strong bonding between
the metal matrix and the hollow spheres is achieved.

[0083] Bonding between the foam components is more clearly evident
in
FIGS. 10-11 which provide SEM images of a cast metal foam according to
the invention comprising hollow low carbon steel spheres surrounded by
an aluminum metal matrix. As can be seen in FIG. 10, the aluminum metal
matrix fills the interstitial space between the hollow steel spheres
with
consistent bonding to the surfaces of the spheres. FIG. 11 provides a
detailed
view of the sphere wall interaction with the aluminum matrix. Very
little
evidence of influx of aluminum matrix into the walls of the hollow
steel
spheres is seen in FIG. 11 indicating low porosity in the wall of the
hollow
steel spheres.

[0084] SEM images of a cast metal foam according to another
embodiment
of the invention are provided in FIGS. 12(a) and 12(b). While it is
preferable
for the interstitial space between the spheres to be completely filled
by the metal matrix, as can be seen in FIG. 12(a), voids can be
present,
particularly at an interface between two spheres. Using geometrical
calculations,
the void space at the interface of two spheres can be calculated
according
to an estimated void angle, and the resulting void volume (V.sub.void)
per contact point of two spheres can be calculated according to the
following
equation:V.sub.void=3.16.times.10.sup.-2(R.sup.3) (1) in which R is the
outer radius of the spheres used in the foam (see Sanders, W. S. and
Gibson,
L. J., Mater. Sci. Eng. A A347, 2003, p. 70-85). Projecting this into a
face-centered cubic (FCC) arrangement of spheres with four spheres per
unit cell, and knowing that in a random loose condition, there are
three
contacts per unit cell, the total volume percentage of voids per unit
cell
can be estimated as: V.sub.vf=(3.16
.times.10.sup.-2R.sup.3.times.12)/22.627R.sup.3
(2) in which V.sub.vf, is the volume fraction of voids. In one
particular
composite metal foam according to the invention prepared by casting
molten
aluminum around hollow low carbon steel spheres, the volume percentage
of voids calculated according to equation (2) was 1.68%. However, the
actual
matrix porosity is expected to be even less, given there are less than
four spheres in each unit cell of the random arrangement and not all
contacts
have a void space. In one embodiment of the invention, the void volume
percentage is less than 1.5%, preferably less than 1.25%, more
preferably
less than 1%.

[0086] As can be seen in FIG. 12 and Table 3, the Al matrix
typically
comprises three different phases. The Al matrix generally comprises
approximately
98% Al. The phase designated the light gray phase is a ternary alloy of
Al, Si, and Fe (estimated to be Al.sub.4FeSi) and is typically found in
two different shapes, plates and needles. The phase designated the dark
gray phase has a composition that is close to the composition of the Al
matrix generally but also includes copper.

[0087] The composite metal foams of the invention (whether
prepared
through casting or powder metallurgy) are particularly characterized in
that they exhibit high compressive strength and energy absorption
capacity
while maintaining a relatively low density. Of course, actual density
of
the finished composite metal foam can be calculated using the measured
sample weight and structural dimensions. It is also possible, however,
to determine an estimated density based on component properties and
packing
properties of the spheres in the mold.

[0088] Sphere packing density is a measure of the relative order
of
the arrangement of spheres, such as in a mold. It is desirable to
achieve
a maximum density of spheres in order to maintain a lowest possible
density
for the prepared composite metal foam and have a uniform distribution
of
spheres, thus contributing to isotropy of mechanical properties. It is
generally recognized that there are three types of packing arrangements
for spheres: ordered packing, random dense packing, and random loose
packing
(See, German, Particle Packing Characteristics, Metal Powder Industries
Federation, Princeton, N.J., 1989). The highest order is represented by
the face-centered cubic (FCC) or hexagonal closed packed (HCP)
structure,
with a 74% packing density of spheres, assuming mono-sized spheres. A
random
dense arrangement has a packing density of 64%. This is achieved by
vibrating
an initially random arrangement to the best attainable packing density.
Random loose packing has a fractional density of 56%-62.5%, with an
average
reported fraction density of 59% and a three-point contact per sphere.

[0089] As previously noted, in one preferred embodiment of the
invention,
after the hollow metal spheres are loaded into a mold (in either a
casting
or powder metallurgy technique), the mold with the spheres is vibrated
to achieved increased packing density, which ultimately leads to
reduced
overall density for the prepared composite metal foam of the invention.
In one test, hollow spheres were poured in bulk into an acrylic box.
Isopropyl
alcohol was then poured into the box as a testing replacement for the
matrix
material to determine the volume needed to fill the box. With this
random
placement, sphere packing density was measured as 56%. In a second run,
the spheres were manually vibrated prior to introduction of the
isopropyl
alcohol. The vibrated spheres exhibited a packing density of 59%.

[0090] The density of a composite metal foam according to the
invention
can be estimated as a function of component density according to the
following
equation: .rho..sub.CF=.rho..sub.SV.sub.fs+.rho..sub.mV.sub.fm (3) in
which
.rho..sub.CF is the density of the composite metal foam, .rho..sub.s is
the density of spheres, V.sub.fs is the volume fraction of spheres (the
packing density of the spheres), .rho..sub.m is the density of the
matrix,
and V.sub.fm is the volume fraction of the matrix. Considering the
effect
of porosity in the wall thickness of metal spheres, equation (3) can be
altered to:
.rho..sub.CF=.rho..sub.metal[1-(V.sub.in/V.sub.out)]V.sub.fs(1-V.sub.fp)+-
.rho..sub.mV.sub.fm (4) in which .rho..sub.metal is the density of the
metal used in the hollow metal spheres (e.g., steel),
V.sub.in/V.sub.out
is the ratio of inner volume to outer volume of the metal spheres, and
V.sub.fp is the volume fraction of porosities in the wall thickness. As
previously noted, the porosity of the walls of the hollow metal spheres
can vary and is preferably less than about 12%.

[0091] As previously noted, the composite metal foams of the
invention
are particularly useful in that they provide a material that combines
strength
with light weight. In particular, the composite metal foams generally
have
a density that is less than the density of the bulk materials used in
the
composite metal foams. For example, steel is generally recognized as
having
a density in the range of about 7.5 g/cm.sup.3 to about 8 g/cm.sup.3. A
composite metal foam prepared according to the present invention using
hollow steel spheres and a steel metal matrix would exhibit a density
well
below these values.

[0092] The composite metal foams according to the present
invention
preferably have a calculable density of less than about 4 g/cm.sup.3.
Preferably,
the composite metal foams of the invention have a density of less than
about 3.5 g/cm.sup.3, more preferably less than about 3.25 g/cm.sup.3,
and most preferably less than about 3.0 g/cm.sup.3. In one embodiment
of
the invention, there is provided a composite metal foam comprising
hollow
steel spheres surrounded by an aluminum metal matrix, the composite
foam
having a density of less than about 2.5 g/cm.sup.3. In another
embodiment
of the invention, there is provided a composite metal foam comprising
hollow
steel spheres surrounded by a steel metal matrix, the composite foam
having
a density of less than about 3.0 g/cm.sup.3.

[0093] The metal foam of the invention can also be evaluated in
terms
of relative density. By analysis of this parameter, it is possible to
compare
the level of porosity of the metal foam (or the level of foaming) with
the level of porosity of the bulk material. According to one embodiment
of the invention, the inventive composite metal foam has a relative
density
(compared to bulk steel) of between about 25% and about 45%.

[0094] The usefulness of the composite metal foams according to
the
invention is particularly characterized by their favorable strength to
density ratio. As used herein, strength to density ratio is determined
as the plateau stress of the metal foam under compression (measured in
MPa) over the density of the metal foam. The composite metal foams of
the
invention typically exhibit a strength to density ratio of at least
about
10. Preferably, the composite metal foams of the invention exhibit a
strength
to density ratio of at least about 15, more preferably at least about
20,
still more preferably at least about 25, and most preferably at least
about
30.

[0095] The composite metal foams of the invention are further
characterized
by improved energy absorption. Energy absorption capability can be
characterized
in terms of the amount of energy absorbed by the material over a given
level of strain. As used herein, energy absorption is defined as the
energy
absorbed (in MJ/m.sup.3) up to 50% strain. The composite metal foams of
the invention typically exhibit energy absorptive capability of at
least
about 20 MJ/m.sup.3. Preferably, the composite metal foams of the
invention
exhibit energy absorptive capability of at least about 30 MJ/m.sup.3,
more
preferably at least about 50 MJ/m.sup.3, most preferably at least about
75 MJ/m.sup.3.

[0097] Any method or apparatus recognizable as useful by one of
skill
in the art for obtaining and analyzing the above-noted properties could
be used and is fully envisioned by the present invention. For example,
SEM images can be obtained through use of a Hitachi S-3200N
environmental
SEM equipped with energy dispersive X-ray spectroscopy. Of course,
other
SEM equipment, as would be recognized as suitable by the skilled
artisan,
could also be used in accordance with the invention.

[0098] One particular method of analysis of the mechanical
properties
of the composite metal foams according to the invention is through
monotonic
compression testing and compression fatigue testing. Exemplary
equipment
useful in such testing is a MTS 810 FLEXTEST.TM. Material Testing
System
(available from MTS Systems Corporation). According to one testing
procedure,
monotonic compression testing is performed using a MTS 810 loading
machine
with a 220 kip load cell. According to another testing procedure,
compression
fatigue testing is performed using a MTS 810 loading machine with a 220
kip load cell having a fixed R-ratio
(R=.sigma..sub.min/.sigma..sub.max)
of 0.1 at a frequency of 10 Hz and an applied maximum stress of 37.5
MPa.

[0100] Monotonic compression testing of all three samples from
FIG.
13 demonstrated the typical behavior of an elastic-plastic foam under
compression.
There is an initial linear elastic region, which is followed by an
extended
region of deformation at a relatively constant level of stress. Unlike
most foams, however, the foams prepared according to the present
invention
do not exhibit a level plateau stress. Rather, the material densifies
at
a slowly increasing rate, and there is no distinct point at which full
densification occurs. As used herein, plateau stress is understood to
refer
to the average stress between the yield point and the point at which
50%
strain (i.e., deformation) has been achieved. All three composite metal
foams shown in FIG. 13 reached a minimum of 50% strain before reaching
a point of full densification.

[0101] FIG. 14 shows stress-strain curves of composite metal foams
according
to various embodiments of the invention under monotonic compression.
Sample
1 is taken from an embodiment formed through powder metallurgy using
3.7
mm hollow low carbon steel spheres and low carbon steel powder. Sample
2 is taken from an embodiment formed through casting an aluminum matrix
around 3.7 mm hollow steel spheres. Sample 3 is taken from an
embodiment
formed through powder metallurgy using 1.4 mm hollow low carbon steel
spheres
and low carbon steel powder. Sample 4 is taken from an embodiment
formed
through powder metallurgy using 2.0 mm hollow stainless steel spheres
and
stainless steel powder. After 50% strain, the composite metal foams
begin
to approach densification as the hollow spheres are completely
collapsed
and the material begins to heave like a bulk material.

[0103] Samples 1 and 3 above are powder metallurgy foams
comprising
low carbon steel. Sample 2 is a cast Al--Fe foam. Sample 4 is a powder
metallurgy foam comprising stainless steel. The comparison in Table 4
indicates
the composite foams of the invention have a noticeably increased
strength
while maintaining a comparable strength to density ratio. Further, the
inventive composite foams show improved energy absorptive properties
making
the composite foams particularly useful in the various applications
described
herein.

EXPERIMENTAL

[0104] The present invention is more fully illustrated by the
following
examples, which are set forth to illustrate the present invention and
are
not to be construed as limiting.

EXAMPLE 1

Composite Metal Foam Prepared by Powder Metallurgy

[0105] A composite metal foam was prepared using stainless steel
spheres
and stainless steel powder according to the specifications provided in
Tables 1 and 2, respectively. The stainless steel spheres had an
outside
diameter of 2.0 mm and sphere wall thickness of 0.1 mm. The spheres
were
cleaned in a solution of 3.0 mL HCl and 97 mL water to remove oxides,
rinsed
in acetone, and dried. A permanent mold made of 304 stainless steel and
having interior dimensions of 5.1 cm.times.5.1 cm.times.10 cm was used.
The mold was prepared by coating its surfaces with a boron nitride mold
release. The spheres were placed in the mold and vibrated for 5 minutes
using an APS Dynamics model 113 shaker and an APS model 114 amplifier
with
a General Radio 1310-B frequency generator. The powder was added and
the
mold was further vibrated to completely fill the spaces between the
spheres.
Total vibration time was 30 minutes at 15-20 Hz.

[0106] The mold was placed in a vacuum hot press during sintering.
Although
no pressure was applied, the mold cap was held in place by the press,
and
the thermal expansion of the spheres was used to create internal
pressure
to aid in the sinter of the powder. The powder and spheres were
sintered
using a 10.degree. C./minute heating rate, held for 30 minutes at
850.degree.
C., further heated at a rate of 5.degree. C./minute and held for 45
minutes
at 1200.degree. C. The mold was then cooled at a rate of 20.degree.
C./minute.
The finished composite steel foam was then removed from the mold for
testing.

[0107] Optical microscopy was performed using a Buhler Unitron
9279
optical microscope equipped with a Hitachi KP-M1 CCD black and white
digital
camera. SEM images were taken with a Hitachi Ss-3200N environmental SEM
equipped with EDX to determine the microstructure and chemical
composition
of the metal foam. Monotonic compression testing was performed using an
MTS 810 with a 980 kN load cell and a crosshead speed of 1.25
mm/minute.

[0108] The composite metal foam had a density of 2.9 g/cm3 (37%
relative
density) and reached a plateau stress of 113 MPa from 10-50% strain and
began densification at around 50% strain. These and further analytical
results are provided in Table 4 (wherein the metal foam from this
example
is shown as Sample 4). FIG. 15 shows a comparison of the stainless
steel
composite metal foam (a) before compression testing and (b) after
compression
testing with 60% strain.

EXAMPLE 2

Composite Metal Foam Prepared by Casting

[0109] A composite metal foam was prepared by casting using low
carbon
steel hollow spheres and a matrix-forming liquid aluminum 356 alloy
according
to the specifications provided in Tables 1 and 2, respectively. The
steel
spheres had an outside diameter of 3.7 mm and sphere wall thickness of
0.2 mm. An open atmosphere gravity feed permanent mold casting system
made
of carbon steel was used, the mold cavity having dimensions of 121
mm.times.144
mm.times.54 mm. The mold was partially preassembled after coating with
a boron nitride powder spray to prevent oxidation to mold surfaces
during
preheating and for providing easy release of the sample after cooling.
The spheres were placed in the mold with a stainless steel mesh to hold
them in place and vibrated for 10 minutes to pack the spheres into a
random
dense arrangement. The mold used was similar to that illustrated in
FIGS.
7 and 8.

[0110] The aluminum alloy was melted in a high temperature furnace
(3300
series available from CM Furnaces) at a temperature of 700.degree. C.
At
the same time, the mold with the hollow spheres inside was pre-heated
in
the furnace to 700.degree. C. to prevent instant solidification of the
aluminum upon contact with the spheres while casting. The fully liquid
aluminum alloy was then poured in the sprue tube of the heated mold.
The
liquid aluminum fills out the cavity while pushing the air out from the
cavity. The filled mold was allowed to air cool, and the mold was
disassembled
and the composite metal foam removed. Testing was performed on the cast
composite metal foam as described in Example 1.

[0111] The cast composite metal foam had a density of 2.4
g/cm.sup.3
(42% relative density). During monotonic compression, the composite
metal
foam reached an average plateau stress of 67 MPa up to 50% strain
before
it began densification at around 50% strain. These and further
analytical
results are provided in Table 4 (wherein the cast metal foam from this
example is shown as Sample 2). Optical and SEM observation indicated
the
Al matrix had nearly filled all of the interstitial spaces between the
steel spheres (see FIG. 12(a)). The void space due to incomplete
filling
of the interstitial space at the sphere point contacts with the Al
matrix
was calculated to be less than 1%.

[0112] The cast composite metal foam was tested to calculate
maximum
cyclic stress. Testing methods are fully described by Banhart, J. and
Brinkers,
W., "Fatigue Behavior of Aluminum Foams", J. Material Science Letters,
18(8), 1999, p. 617-619, and Lehmus, D., et al., "Influence of Heat
Trea.mu.ment
on Compression Fatigue of Aluminum Foams", Journal of Material Science,
37, 2002, which are incorporated herein in their entirety. The average
yield strength calculated, using the 0.2% offset method, was 29 MPa.
The
maximum stress was chosen to be 85% of the reference strength. The
fatigue
was continued with this maximum stress for 250,000 cycles with no
apparent
deformation. The maximum stress was then increased to 37.5 MPa (the
stress
at 5% strain from the stress-strain curve). FIG. 16 shows the curve for
the compression fatigue test as the relation between strain and the
number
of cycles.

[0113] The cast composite foam deformed by an initial progressive
shortening,
followed by collapse of the spheres starting at certain regions with
possible
defects like holes or cracks, causing the subsequent failure of the
neighboring
spheres leading to the formation of macroscopic collapse bands. Visual
observation of the deformed fatigue cast foam revealed that extensive
fatigue
failure had occurred within the crush bands. The S-N curve (FIG. 16)
shows
the initial progressive fatigue damage at the onset of an abrupt strain
jump. The cast foam sample had endured 1,440,000 cycles before the end
of the fatigue test. Optical images of the cast foam taken before and
during
the fatigue test are shown in FIG. 17.

[0114] Many modifications and other embodiments of the invention
will
come to mind to one skilled in the art to which this invention pertains
having the benefit of the teachings presented in the foregoing
descriptions
and associated drawings. Therefore, it is to be understood that the
invention
is not to be limited to the specific embodiments disclosed and that
modifications
and other embodiments are intended to be included within the scope of
the
appended claims. Although specific terms are employed herein, they are
used in a generic and descriptive sense only and not for purposes of
limitation.